implantable devices that provide electrical stimulation to selected chambers of the heart have been developed in order to treat a number of cardiac disorders. A pacemaker, for example, is a device which paces the heart with timed pacing pulses, most commonly for the treatment of bradycardia where the ventricular rate is too slow. Implantable devices may also be used to treat cardiac rhythms that are too fast, with either anti-tachycardia pacing or the delivery of electrical shocks to terminate atrial or ventricular fibrillation,
Implantable devices have also been developed that affect the manner and degree to which the heart chambers contract during a cardiac cycle in order to promote the efficient pumping of blood. In a properly functioning heart, contraction of each atrium or ventricle is synchronized with the contralateral atrium or ventricle, Without such synchronization, the heart's pumping efficiency is greatly diminished. To treat patients suffering from inefficient or unsynchronized pumping of the heart, CRT devices, which provide appropriately timed electrical stimulation to one or more heart chambers in an attempt to improve the coordination of atrial or ventricular contractions, have been developed.
A common form of CRT applies stimulation pulses to both ventricles, either simultaneously or separated by a specified biventricular offset interval, and after a programmed atrio-ventricular (AV) delay interval with respect to the detection an intrinsic atrial contraction or delivery of an atrial pace. With such CRT, an electrode of a right ventricular lead is placed in contact with the right atrium, typically at the apical wall, and a left ventricular lead containing an electrode for pacing the left ventricle is typically placed in a vein of the coronary sinus overlying the left ventricle.
Typically, it is desirable to pace the ventricles before intrinsic contraction of the ventricles occurs. In cases where an intrinsic ventricular contraction is sensed, a biventricular pacing is often applied soon thereafter, or simultaneously therewith, to produce an effective ventricular contraction. Biventricular pacing on a sensed ventricular contraction is commonly referred to as a ventricular sense response (VSR).
Current CRT devices do not employ a metric for evaluating the efficacy of VSR as compared to standard bi-ventricular pacing. While VSR is intended to generate a composite beat comprising paced and non-paced (intrinsic or otherwise) activation wavefronts, methods for determining the effectiveness of the VSRs are lacking. In general, the less the similarity between the VSR beat and a pure biventricular pacing beat, the less effective the VSR.